Red clays indicate sub-aerial exposure of the Rio Grande Rise during the Eocene volcanic episode

Autonomous underwater vehicle (AUV) mapping of the western Rio Grande Rise (RGR), South Atlantic, and subsequent exploration and photography of horizontal lava flows exposed in near vertical, faulted escarpments, showed occurrences of red clays/weathered volcanic tops trapped between successive alkaline lava flows. These red clays indicate a hiatus in successive volcanic eruptions. Here, we report detailed mineralogical, geochemical, and rock magnetic characteristics of one such distinct red clay dredged from ~ 650 m water depth in the western RGR. The mineral constituents of the red clay are kaolinite, magnetite, oxidized magnetite (/maghemite), hematite, and goethite, with biogenic calcite and halite occupying voids or precipitated on the surface of the red clay. The chemical index of alteration (CIA) has a value of 93, showing that red clay is a product of extreme chemical weathering of the lava flows. The alkaline volcanic rocks recovered from nearby show an age of ~ 44 Ma, indicating an Eocene age for the volcanism. We show that the red clays are a product of sub-aerial chemical weathering of these Eocene volcanic rocks, in a warm-wet climate, before the thermal subsidence of the RGR to its modern-day bathymetric depth.

Several alkaline rocks also showed a hump-like signature between ~200 and 400 °C, which might indicate titanomagnetite/titanomaghemite minerals 47 or aggregates of the fine magnetic particles superparamagnetic (SP)-SD produced during alteration 48 .The RGR1_D05_001 (trachyandesite) showed unique and non-reversible curves with cooling curves having higher susceptibility indicating neo-formation of strongly magnetic minerals during the heating.This sample also has the highest CIA value = 78, compared to the other volcanic rocks, and shows maximum alteration (Table S3).
The hysteresis data showed narrow constricted closed loops for most of the volcanic rock samples acquiring saturation well below 500 mT indicating dominant coarse-grained ferrimagnetic mineral composition (Fig. S5).The RGR1_D05_001 (trachyandesite) sample showed relatively higher coercive force compared to the other volcanic rocks, and Mrs/Ms = 0.58 and Hcr/Hc = 1.19 values indicate single domain particle.Several trachyte rocks showed multi-domain behavior with the symmetrical upper and lower half of the FORC diagrams, low coercivity (Hc = <20 mT at the peak of the distribution), and vertical axis spread (Hu <40 mT) diverging away from the Hu = 0 axis (Fig. S6).The picro-basalt and trachybasalt (RGR1_D08_004 and RGR1_D13_002) showed asymmetrical upper and lower half of the FORC diagrams with low vertical spread (Hu <20 mT), typical of the PSD grains (Fig. S6).
The RGR1_D05_001 (trachyandesite) showed most unique FORC diagram with very strong negative interaction up to 80 mT on the Hc axis, while the Hu axis ranges from a few mT to 15 mT (Fig. S6).This sample showed SD magnetic particle behavior.Table S1.Location and water depth of red clay and volcanic rocks dredged during the R/V Alpha Crucis expedition in 2018.The rock type information is based on the detailed petrology and trace element analysis provided in Guerra et al. 43      For the sample description see Table S1.S1.S1.

Supplementary Figures
Figure S1.SEM image and EDX point analysis of the groundmass, lithic fragments and foraminifera assemble in the red clay sample.The foraminifera embedded in clay indicates its post-clay formational deposition.

Figure S2 .
Figure S2.SEM image and EDX point analysis of the groundmass and lithic fragments along a weathering related fracture in the red clay sample.

Figure S3 .
Figure S3.SEM image and EDX point analysis of the groundmass and coarse lithic fragments in the red clay sample.The coarse lithic fragments is composed of relatively weathering resistant minerals e.g., Fe-Ti oxides (titanomagnetites).

Figure S4 .
Figure S4.The χ-T results of the different alkaline volcanic rocks dredged from the WRGR.

Figure S5 .
Figure S5.Hysteresis results of the alkaline volcanic rocks.The Mrs/Ms and Hcr/Hc values are also provided.For the sample description see TableS1.

Figure S6 .
Figure S6.FORC results on different alkaline volcanic rocks recovered from the WRGR.For the sample description see TableS1.

Table S2 .
. The XRD results (d spacing) of bulk red clay sample.

Table S3 .
Major element chemistry and loss on Ignition data (LOI) data for different alkaline volcanic rocks and basalts (DSDP 516) used for the calculation of chemical weathering index (CIA).The sample details and data are after Hoyer et al.10and Guerra et al.43.